Demonstrating low Raman background in UV-written SiO2 waveguides

Abstract

Raman spectroscopy can give a chemical ’fingerprint’ from both inorganic and organic samples, and has become a viable method of measuring the chemical composition of single biological particles. In parallel, integration of waveguides and microfluidics allows for the creation of miniaturized optical sensors in lab-on-a-chip devices. The prospect of combining integrated optics and Raman spectroscopy for Raman-on-chip offers new opportunities for optical sensing. A major limitation for this is the Raman background of the waveguide. This background is very low for optical fibers but remains a challenge for planar waveguides. In this work, we demonstrate that UV-written SiO₂ waveguides, designed to mimic the performance of optical fibers, offer a significantly lower background than competing waveguide materials such as Si₃N₄. The Raman scattering in the waveguides is measured in absolute units and compared to that of optical fibers and Si₃N₄ waveguides. A limited study of the sensitivity of the Raman scattering to changes in pump wavelength and in waveguide design is also conducted. It is revealed that UV-written SiO₂ waveguides offer a Raman background lower than −107.4 dB relative to a 785 nm pump and −106.5 dB relative to a 660 nm pump. Furthermore, the UV-written SiO₂ waveguide demonstrates a 15 dB lower Raman background than a Si₃N₄ waveguide and is only 8.7 − 10.3 dB higher than optical fibers. Comparison with a polystyrene bead (in free space, diameter 7 µm) reveal an achievable peak SNR of 10.4 dB, showing the potential of UV-SiO₂ as a platform for a Raman-on-chip device capable of measuring single particles.